Can you trust the site? Can you trust the opened product hasn't been installed and activated on someone else's pc? Personally, I'd spend the extra $$ for an un-opened product (that's just me), but having said that, maybe it's perfectly ok, in which case it is a good deal. Trouble is, how can you know until you get it?

If you purchase from a credible retailer an un-opened, genuine product that you can actually get your hands on before the purchase is made, then you that's the best confirmation possible. There's MS Store as well: